Control circuit



April 1950 P. c. MICHEL ET AL 2,505,565

CONTROL CIRCUIT Filed Nov. 14, 1946 2 Sheets-Sheet l Inventors: Philip C. Michel, Theodorre A. Rich,

Their A'tt orney.

April 25, 1950 lCHEL ETAL 2,505,565

CCCCCCCCCCCC IT Fig. 3.

MMMMMMY VVVVVVVVVVVU JW WWW Irwverwtors: Philip C. Michel, Theodore A. Rich,

Their Attormey Patented Apr. 25 1956 UNI-TED STATES ENT OFFICE CONTROL CIRCUIT Application November 14, 1946, Serial No. 709,738

ZClaims. 1

Our invention relates to control circuits and more particularlyto a new and improved control circuit utilizing a radio frequency bridge circult in such fashion that not only the degree of unbalance but also the direction of unbalance causes the control elements to operate a manner whereby accelerated action is accomplished without introducing hunting.

Control circuits employing bridge circuits which utilize an A. C. source of the same frequency as the source from which the device to be controlled is energized have been used previously with success. However, the accuracy with which they may be made to operate is subject to the effects of stray field's or magnetic linkages between the energizing circuit of the load apparatus and the control circuits. Where accurate control is desired these effects must be minimized which is not possible in most cases without utilizing extensive shielding which itself is otherwise a disadvantage. Control circuits employing radio frequency bridge circuits are also used and since they operate at a frequency far different from that of the source energizing the load equipment electromagnetic or electrostatic shielding is no longer a factor in determining accuracy. However, the usual type of circuit utilizing a radio frequency bridge circuit usually is sensitive only tounbalance of the bridge circuit and auxiliary methods must be employed to determine the direction of unbalance. It is desirable that a circuit be devised to operate in conjunctionzwith a radio-frequency bridge in such fash on that control is exercised upon the load circuit in a manner dependent not only upon the degree of unbalance but also upon the direction of unbalance.

It is an object of our invention to provide a new and improved control circuit utilizing a radio frequency bridge circuit which produces an output dependentv upon. both the degree of radio frequency bridge and whose. phase relationship with. respect to a low frequency source voltage is dependent upon the direction of such unbalance.

It is a still further object of our invention to provide a new and improved control circuit employing a radio frequency bridge circuit whose output is a D. C. voltage comprising a component of fixed amplitude and polarity and a component whose amplitude is dependent upon the degree of unbalance and whose polarity is dependent upon the direction of unbalance of the radio frequency bridge circuit.

The features of the invention which are believed to be novel and patentable will be pointed out in the claims appended hereto. For a better understanding ofthe invention, reference is made in the following description to the accompanying drawing in which Fig. 1 discloses a control circuit for regulating the temperature of an electrically heated oil bath; Fig. 2 presents curves of voltages existing in portions of the circuit of Fig. 1 when unmodulated radio frequency voltages are utilized, and Fig. 3 presents similar curves for the case when amplitude modulated radio frequency voltages are utilized.

In general, the circuit of Fig. 1 discloses a power supply, as shown in the upper left-hand portion of the figure, an essentially constant amplitude, constant frequency oscillator including tube l5 and associated elements, an amplitude modulating stage utilizing tube 3|, and an alternating current bridge supplied with amplitude modulated radio frequency voltage. The bridge supplies an output voltage of a magnitude and phase relationship with respect to the impressed radio irequency voltage dependent upon the degree and direction of unbalance of the bridge. This voltage is amplified by tube 54 whose associated elements, including tube "i3, cause it to possess automatic volume control features.

The amplified output of tube 13 is added to a constant amplitude radio frequency voltage derived from the radio frequency oscillator, and the resultant voltage is rectified by tube 8b. This r ctified voltage is, in turn, utilized to control a phase discriminatory circuit comprising tubes 85 and t? which, in turn, operate by phase control on tubes l l I and I I2 to regulate current flow in the circuit to a heater A. Additional heaters may be controlled by relays actuated by current flow to heater A, which current flow is dependent upon both the degreeand the direction of bridge unbalance in combination with relays actuated by the degree of bridge unbalance only, as eviaccuse denced in the amplifier circuit including tube 54. Thus, the described control circuit is capable of imposing conditions dependent upon both the degree and the direction of unbalance.

In Fig. 1, the control circuit is supplied from an A. 0. power source which may also be the source for the electrical heating elements of the oil bath or other load to be controlled. This source energizes a transformer I by proper connections to a primary coil 2 of transformer i. Transformer I is provided with at least three tapped secondary coils 3, 4 and 5 and may also possess an additional secondary coil, not shown, for supplying voltage to the cathode heaters of the various tubes utilized.

A. D. C. anode voltage source is provided in a customary manner by utilizing the center tapped secondary coil 4 and a double anode full wave rectifier tube 6 whose anodes I and 8 are connected to the respective ends of secondary coil 4.

Cathode 9 thereby is maintained positive with respect to the center tap of secondary coil 4 and a filter circuit consisting of an inductance or choke coil it and condensers II and I2 is utilized to filter out the cathode pulsations by connecting cathode 9 to one side of a choke coil I and connecting condenser II between cathode 9 and the center tap of secondary coil 4 and connecting condenser I2 from the other end of coil It to ground. The center tap of secondary coil 4 is in turn connected to ground through a tapped resistor l3 which is shunted by a radio frequency by-pass condenser 54. It will be evident to those skilled in the art that this part of the circuit is a conventional D. C. power supply comprising a fullwave rectifier and filter, and that other power supplies capable of providing the required D. C. voltages and currents may be substituted without changing the operating principles of our invention.

A normally conducting pentode oscillator tube has its cathode I 6 connected to an appropriate tap on an inductance coil 2! and its control grid I! connected to one end of coil 2! through a grid resistor 22 which is shunted by a condenser 23. Coil 2! is connected to ground at the other end and is shunted by a fixed tuning condenser 24. Positive voltage from tube 6 is impressed upon the screen grid I8 of tube I5 through a resistor 25 and a radio frequency by-pass to ground is provided by a condenser 26. Suppressor grid i9 is connected to ground while anode 20 is connected to the positive potential supplied by tube 6 through the serially connected resistor 2! and a tuned circuit consisting of an inductance coil known in the art will also give good results.

The radio f equency voltage output of tube I5 is amplified and amplitude modulated by a normally conducting tube 3| whose cathode 32 is maintained at ground potential as is also its suppressor grid 35. Control grid 33 connected to ground through a grid resistor 3'! is capacitively coupled to the anode 20 of tube I5 by means of a coupling condenser 38. Screen grid 34 is held at a positive potential by connection to the D. C. power supply through resistor I39. A radio-frequency by-pass capacitor 39 is provided to prevent radio-frequency variations in the potential of screen grid 34. Anode 36 is connected to one side of a tuned radio frequency circuit consisting of an inductance coil 40 shunted by a condenser M and is supplied with a voltage having both D. C. and A. C. components by connecting the other side of the tuned circuit to the positive voltage of tube 6 through the secondary coil 3 of transformer I. A radio frequency choke coil 42 is inserted between coil 3 and the tuned circuit 40 and 4! which in cooperation with a condenser 43 connected between the tuned circuit side of coil 42 and ground prevents the radio frequency oscillations of coil 40 from causing radio frequency voltages across the secondary 3 of transformer I. Thus the radio frequency voltage of coil 40 is modulated in amplitude by the voltage of secondary coil 3 of transformer I at power source frequency. The degree of modulation may be controlled by manipulation of a switch 44 or may be made zero by causing switch 44 to contact terminal 45. This part of the circuit is a plate-modulated radio-frequency amplier stage. Stages employing other forms of amplitude modulation may be substituted with good results.

An alternating-current bridge circuit is energized by the amplitude modulated radio frequency voltage output of tube 3| which is made available across an inductance coil 46 by virtue of its inductive coupling to coil 40. A condenser 41 connected across coil 46 is employed to tune coil 45 to the frequency of coil 40. Coil 46 is connected across the bridge in such fashion that radio frequency voltage is impressed across a serially connected resistor 48 and manually adjustable condenser 49 and also across a serially connected resistor 5i] and temperature responsive condenser 51. A voltage caused by bridge unbalance exists between the junction of resistor 48 and condenser 49 and the junction of resistor 59 and condenser 5|. The tuned circuit consisting of an inductance coil 52 shunted by a condenser 53 is subjected to the unbalance voltage of the bridge by proper connections thereto.

A normally conducting tube 54 whose cathode 55 and suppressor grid 58 are maintained at ground potential operates as a radio frequency amplifier subject to the automatic gain control features introduced by a tube 13 as will be described. Control grid 56 is connected to one side of an inductance coil 60 which is shunted and tuned by a condenser 6|. Coil 60 is inductively coupled to coil 52. The other end of coil 60 is capacitively coupled to ground through a condenser 62 and is normally maintained at a negative potential with respect to ground by its connection to an appropriate tap on resistor I3 through serially connected resistors 63, 64, and 65. The positive potential at tube 6 is supplied to screen grid 5! of tube 54 through a resistor 63 and the electromagnetic coil of a relay 5'! connected in series. Screen grid 51 is also capacitively coupled to ground by a radio frequency bypass condenser 68. Anode 59 of tube 54 is connected to screen grid 51 through a tuned circuit consisting of a coil 69 shunted by a tuning condenser It. The output of tube 54 is made available across an inductance coil II inductively coupled to coil 69. Coil I! is tuned to the proper It to the ungrounded side of coil I-I.

.means of. a condenser Il.I..

accuses capacitively connected to ground by a condenser 16. Condenser: "I6 has suificient capacity with respect to the. resistance, value of resistor 65 *plus a portion. of resistor I3 which in series constitutes a, discharge path for condenser 16 in order'to maintain an essentially constant negative D. C. voltage on anode I5 of tube I3. Ad'- diti'onal capacity to ground is supplied by condenser 'I 'I coupled betweenground and the junc- "tion oriresistors '64 and 63.

It should be noted that tube- Hdoesnetconduct until the R. F. volt age applied thereto exceeds the D. C. bias pro- 'vided by a portion of resistor [3. Tube I3 thus providesa delayed automatic-gain=control action to amplifier tube '54. Other amplifier circuits having automatic-gain-control features may also be employed.

A radio frequency voltage of essentially constant amplitude derived from the oscil ator tube I5 is added to the output radio frequency voltage of tube 54- by connecting one side of a coil Coil I8 is inductively coupled to coil 28 and is tuned to the proper radio frequency by a shunting condenser l-i-i' The algebraic sum of the voltages of coils II and I8 is impressed across a diode rectifier tube 89 by connecting the anode 8| of tube 86 to one side of coil 18', the other side of which is cono nected to the ungrounded side of coil II, and by connecting cathode 82 of tube 80 to ground through a resistor 83 shunted by a condenser 84. Resistor 83 is also shunted by a condenser 84' and a resistor 85 in series and the voltage across 4' condenser 84' thereby contains a smaller radio frequency component than does resistor 83 or condenser 84. Tube Bil acts as a detector or demodulator.

When an amplitude modulated radio frequency input is. impressed upon the bridge circuit of. resistors 4'8 and 50 and condensers 49 and 5! a source frequency A. C. voltage appears across condenser 84" whenever the bridge is unbalanced and this voltage is either in phase 01' 180 out of phase with the source voltage depending upon the direction of unbalance on the bridge.

A normally conducting tube 86 with a cathode 8?, grid 88 and anode 89 is employed as a mixer or voltage adder wherein a resultant voltage isobtained the phase of which is dependent upon the relative magnitude of. two A- C. voltages of the same. frequency but displaced 90 in phase with respect. to each other. Control grid 88 or tube 86. is capacitively coupled. through. aresistor.

90. to the'unground'ed side. of. condenser 84' by Grid 88 is also connected to ground through resistor 99 by means of a resistor 92. Anode89, of tube 86 is connected to the. positivev potential available at tube 6 through a resistor 9.3. Cathode 8'! through a resistor is subjected to an A. C. voltage with respect to ground by a connection to the Junction. of a resistor 95 and a condenser 96 which elements. are. connected in. series across the teriii) minals of. the secondary coil 5 of transformer I. The-center tap of winding 5 lsgrounded. Resistor 95 and condenser 96 operate as a phase shifter which is designed to shift the phase by substantially 9.0 for reasons to be explained.

A normally conducting tube 91 with a cathode 98,, a control grid 99, and an anode I8!) is employed in a circuit from which two A. C. voltages of equal amplitude. but out of phase with each other may be obtained. Thus tube 91 is an inverter used to obtain a push-pull output.

Cathode 9B is connected to ground through two serially connected resistors IGI and I02 while anode I69 of tube 9-! is connected to the plate voltage supply through resistor I03. Control grid '99: of tube 9! is capacitively coupled to anode 89 of tube '86 by means of a condenser I84 and is connected to ground through two serially connected resistors I85 and I82.

The circuit just described may be employed to control the temperature in an oil bath I06 by placing the temperature responsive condenser 5I therein and by supplying heat energy thereto by means of electrical heating units A, B, and C. These may be energized from the same A.. C. power source as employed for transformer I, either through a transformer I0! or directly at least in part. Primary winding I83 of transformer I01 is energized from the A. C. source while the tapped secondary coils I09 and III] are utilized in a manner to be described.

The. center tap of a secondary winding I09 is connected directly to a terminal of an electric heating unit such as heater A which is preferably of smaller heat producing capacity than any of the other heating units in oil bath I06. The other terminal of heater A is connected to the cathodes of, two grid controlled mercury vapor discharge tubes HI and H2 and to ground. Anodes I13 and IM of tubes HI and H2 respectively are connected to respective end terminals of secondary coil I69 of transformer I81. The control grids I i 5 and N6 of tubes I I I and 5 I2 respectively obtain a normally negative bias by connecting each to the same appropriate tap on resistor I3 through their respective grid resistors I I! and H8. Control grid H5 is, in addition, capacitively coupled to cathode 98 of tube 91 by a condenser H9 and similarly grid H6 is capacitively coupled to anode Hill of tube 91 by a condenser I20. The electromagnet winding of a relay I2I is connected across heater A and in parallel with it.

A lead I22 from one terminal of the A. C. power source extends directly to one terminal of an electrical heating unit such as heater C which may be preferably the unit possessing the greatest heat producing capacity. A lead I23 eXtends from the other terminal of the A. C. power source to a contact point I25 of a three position manually operated switch H4. The arm of switch I24 may be made to contact any one of three contacts E25, I25, and I 21. The remaining terminal of heater C'is connected. to the contact arm of switch I24.

The electromagnet coil of a relay 28 is coni nested between lead i 23 and contact point we of energized to allow its plunger to fall. Heater B is connectedbetween lead I22 and contact arm I26 of relay I28 which arm may contact either contact point I32 or I33. Contact arm I39 of relay I28. is connected to contact point E25 of the manually operated switch I24. Arm IBBwil'l contact point l3l when relay I28 is energized. Contacts HI and use of relay H28 and contact lSfi of relay l2! are all connected together and contact arm l3! of relay {2! is connected to power lead I23. That portion of secondary coil its of transformer llll chosen by adjustment of a manually operated switch l38 is connected between lead I22 and contact point 832 of relay I28.

The operation of the control circuit in automatically maintaining the oil bath temperature constant may be described in the following manner. D. C. voltage requirements of the circuit are met employing a rectifier tube 6 and its associated elements in cooperation with secondary coil 4 of transformer l whereby the ungrounded terminal of condenser 12 may be considered a source of D. C. anode voltage and the voltage drop through resistor it causes its ungrounded end to become negative thereby allowing tapping of resistor l3 for obtaining appropriate negative grid .1

biasing voltages.

Tube is and its associated elements constitute an electron-coupled Hartley type radio frequency oscillator which by design may be made to oscillate at the desired radio frequency. Tube is will cause oscillations of essentially constant amplitude affected only negligibly by the temperature characteristics of the tube and its associated elements. In the circuit includin tube 3!, the constant amplitude radio frequency voltage output of tube I is amplified and amplitude modulated at A. C. source frequency by imposing an A. C. component of voltage upon anode 36 of tube 3! by means of the secondary coil 3 of transformer l in series with the D. C. supply lead to anode 3G. 1

The degree of amplitude modulation may be manually adjusted by proper positioning of switch t l and in fact may be entirely eliminated by turning switch 44 to contact point 45.

The radio frequency output voltage of tube 3| as illustrated by curve b of Fig. 2 for unmodulated and curve b of Fig. 3 for modulated radio frequency voltage is made available across the coil 45 which is inductively coupled to coil 40 in the anode circuit of tube 3!. This voltage is impressed across a resistance-capacitance type alternating current bridge, one branch of which contains a resistor 48 and a manually adjustable condenser 49 and the other branch of which contains a resistor 50, either equal to or some desired proportion of resistor 48, and a condenser 5! of such a characteristic that its value is dependent upon its temperature. When the bridge is balanced, the voltage across the terminals separated by resistors 18 and 5B in series and by condensers 49 and 5! in series is zero. If an unbalance occurs, as by a change in the value of condenser 5i because of temperature change, a voltage exists between the above mentioned terminals which voltage is either substantially in phase with or substantially 180 out of phase with the voltage impressed upon the bridge. This voltage depends upon direction of unbalance for phase determinatime and upon the degree of unbalance for its amplitude. This is illustrated by curve 0 of Fig. 2 or 3 for one direction of bridge unbalance, and by curve d of Fig. 2 or 3 for the opposite direction of bridge unbalance. It is preferable that con-denser 59 present a reactance at the radio frequency of the voltage of coil 46 which is essentially equal to the resistance of resistor 48 and a like relationship should exist between resistor and condenser El in order to insure that the substantially in-phase or substantially 180 cut-of-phase relationship described may exist even for relatively large unbalance of the bridge. Bridge cit-'- cuits containing other types of impedances such as resistances, inductances or capacitances or combinations thereof may also be used with the control circuit described herein.

The radio frequency voltage caused by unbalance of the bridge circuit is impressed upon the control grid circuit of the amplifier tube 54. The control grid 56 of tube 54 is normally biased negatively to an extent dependent upon the positioning of the tap on resistor 13. This condition exists at zero input signal voltage or at such time when the bridge circuit is balanced. At normal bias the D. C. anode current of tube 54 is of suflicient size to cause the electromagnet coil of relay 51 to attract and hold the magnetic plunger and its associated contact arm in the energized position, as shown in the figure, such that arm I35 remains separated from contact point I34. When the bridge circuit becomes unbalanced in either direction a radio frequency voltage appears across coils 52 and 6G and by amplifier action of tube Si t similariy across coils fed and N. If the amplitude of the radio frequency voltage in coil H exceeds the normal bias voltage the diode rectiiier tube it causes condenser 16 to become more negatively charged than previously thereby increasing the negative bias on tube 54.

The increase in negative bias of tube 54 not only decreases the effective amplification of tube 5 since tube '53 operates as an automatic volume control for tube 54, but in addition the D. C. anode current drawn by tube 54, which flows through the electromagnet coil of relay 6?, is also ecreased. Relay 5'! is preferably of such characteristic that its plunger is released by the electromagnet coll thereby allowing arm I35 to contact point i3 4 at a predetermined current value equal to the value to which the anode current of tube 54 is decreased by action of tube 13 for a chosen degree of unabalance of the bridge circult.

The operation of the circuit consisting of the serially connected coils l8 and 'H connected across tube 853 and its cathode resistor 83 which is shunted by condenser 84 may be explained in the following manner. A constant amplitube radio frequency voltage illustrated by curve a of Fig. 2 or Fig. 3 exists across coil l8 because of its inductive relationship to the tuned plate circuit coil 28 of oscillator tube it. The amplitude of this voltage is made suiiiciently large so as to exceed the voltage induced in coil H for even the greatest contemplated degree of unbalance in the bridge circuit. By serially connecting coils Z8 and H the voltage of coil 7| is added algebracially to the voltage of coil 18. The voltages in coils l8 and H are at identical frequencies and are either in phase or are 180 out of phase with each. other. Thus for an unmodulated voltage across coil H the voltage across the coils inseries will possess a constant amplitude exceeding the voltage of coil F8 if the bridge unbalance is in such direction as to cause the voltage of coil H to be in phase with the voltage of coil '58 and the amount of such excess wiil be dependent upon the degree of unbalance of the bridge circuit. This condition is illustrated by curve E of Fig. 2. Similarly, unbalance in the opposite direction causes the voltage across the coils in series to possess a constant amplitude less than the voltage of coil 78 to an extent dependent upon the degree of unbalance as illustrated by curve f of Fig. 2. By rectifying action of diode tube 80 the resultant voltage will appear as a D. C. voltage across condenser 84 and resistor 83 in parallel whose valu is equal tothe amplitudeci theresultant radio frequency voltage; Thus. condenser B l will be normally charged toa D. C. value equal to the amplitude of the voltage of: coil iii and willbecome charged to ahigher value during unbalance in one direction orcharged toa lower value for unbalance in the opposite direction. as. illustrated by curve g of Fig. 2.

Whentheoutput voltage appearing across coil TI is an amplitude modulated radio frequency wave, similar conditions as. toa resultantvoltage across coils H and i3 in series apply as before. Thus, curve 8 of 3- illustrates unbalance in one directionand curve 1 of. Fig. v3 unbalance in the opposite direction. The resultant voltage, however, will be an amplitude modulated wave thereby causing the voltage across condenser 34 to possess a fixed D. C, component plus an A. C. component atpower source-frequency whose amplitude is dependent upon. the degree of unbalance of the bridge circuit. Further, since the instantaneous voltage of .coil. 75 is of the same polarity as thatof the instantaneous voltage of coil '53 during the total radio frequency cycle and all such cycles when the bridge circuit is unbalancedzinone direction andsimilarly of opposite polarity when the bridge circuit is unbalancedxin. the opposite-direction, whether the voltage of coil. .l-iiis amplitude modulated or not, it can. be shown. that. the A. C. component at power, source frequency. appearing across con denser tiluisinphase With;the A. C. power source reference voltage when. thebridge circuit is unbalancedin misdirection. as. illustrated by solid linecurve of Eig. 3g,and:is 180 out :of phase with such reference voltagewhenthebridge circuitis unbalanced. in, the opposite direction, as illustrated by thedotted line curve of Fig; 39., Since condenser dl. WilLcharge up to the average D. C. Vol a e value of condenser 84' whose. charge is essentially equivalent. to condenser 8.4, only the A. Cucomponent f voltage on condenser 84 or of il lwillappear across resistor 92,.

The normally. conducting triode tubeBfi and its associated; elements constitute an. automatic phase shitting device designed in such fashion that theA; outputvoltage of anode slleads or lagsareference voltage such as the A. C. power source voltageto an extent dependent upon the amplitude and phase relationship of the A. C. outputvoltage or" tube 89 or its equivalent, the voltage of. condenser 5 5.; With no A. C. voltage across condenser as, tube 86 operates in such manner as to'supply an A. C. voltage to grid 99 of'tube 9i essentially 90-behind-the A. C. power source reference voltage. When the bridge circuit is unbalanced because'the temperature of the oil bath is low an A. C. voltage appears across resistor 92 and tube 86 causes the A. C. voltage on; grid 99 of tube ill-to be less than 90 behind the A; C; power source reference voltage. If the bridge circuit is unbalanced in the opposite direction, asfor example by excessive temperature in th oil bath, the A; 'C'. voltage of grid 99 of tube 91 is caused by action of tube 8% to be more than 90 behind the reference voltage.

Normally conducting tube 9'! is employed in a circuit such that the voltage of cathode 98 follows the A. C. control grid voltage on grid 99 by cathode follow-er action and is in phase with it while the voltage of anode N10 has an A. C. component equal to but 180 out of phase with the A. C. variation in voltage of cathode 98. This equality of amplitude is accomplished by using a value of i0 resistance; for resistor I93 equal to the sum of resistors it! and. m2.

When an A. C. power source is applied to transformer H17, a: pulsating unidirectional current is supplied to heating unitA through the mercury vapor discharge tubes H i. and iii. Theaverage valuev of this. current and. therefore. the average voltage across heaterA may be regulated by exercisin proper gridcontrol on tubes 1 l i and H2. In normal operation when the bridge. circuit of the. control device is balanced, each control grid is subjected to aproper negative D. C. bias and to an rr..C..voltage.which lags by essentially the A. vcltageon their respective anodes thereby causing. each tube'to be conductive alternately during essentially. one-half the interval of each cycle when its respective anode is positive. If the bridge becomes unbalanced as by. a low temperature in the oil bath, the phase of. each grid will be advanced so that each tube becomes conductive during alonger interval of each cycle thereby increasing the current to and voltage across heater A. Conversely, abridge unbalance caused byexcessive temperature in the oilbath decreases the time intervalin each cycle during which tubes ill and H2: are conductive thereby decreasing current now to andthe voltage across heater A. Thus the heating rate of heater A is controlled by. temperature chan es in the oil bath. In addition, relay IN is designed to remain unactuated when the current flow to heater A corresponds to bridge balance but is actuated orclosed when the current flow to heater A exceeds the value at bridge balance by. apredetermined amount.

At bridge balance, relays t1; i2i, and I28 are in the position shown on the drawing and switch I25 is preferably .-in theposition shown. Contact arm 29 is. incontact with point I32 andheater B is beingenergized by coil H0 of transformer iii? to an extent manually controlled by the position of switch i38. It is desirable to adjust switch I33 so that'normalconduction and radiation heat losses fromthe oil bath are. essentially balanced by heat energy supplied at bridge balance by heaters Aand'B.

If the bridge circuit becomes unbalanced in either direction to a predetermined amount, relay til-drops out and arm l35 contacts point I3 3 since at unbalance the D. C. anodecurrent of tube 54 isdecreased by the increase in negative bias on tube 54 as-caused by action of tube '53. Relay lid-is thereby energized by the A. C. power source and arm129-contacts point I33 and arm I30 contacts point I31.

Ifthe unbalance which causedrelay Bl to drop out: is caused by excessive temperature, the current supplied to heater A is decreased as explained previously and relay-l2] remains unenergized. Thus-heaters i3- and C are deenergized and heater A is supplying heat energy at a below normal rate. If, however, the unbalance causing relay W to drop out is causedby low oil bath temperature the current to heater A is above normaland'relay I2! is energized suificiently to opcrate therebygeifectively connecting" points it! and l33=to lead'l23". Therefore, for low temperatures heater A produces heat at an above normal rate and both heaters B and C produce heat at their maximum rate since each is connected directly across the A. C. power source.

Amon the advantages of the control and energizing circuit herein disclosed the principal advantage in addition to its freedom from power source frequency inductive interference eiiects is that the proper desired temperature may be reached quickly without overshooting or hunting. At an excessive temperature heaters B and C are deenergized and heater A produces heat at a below normal rate. As the desired temperature is approached but before it is reached relay 9'! is actuated and heater B again supplies heat at a rate dependent upon the adjustment of switch 138 thereby decreasing the rate at which the temperature decreases to the desired value. Further, at a low temperature heaters 13 and C supply heat energy at a maximum rate and heater A is operating at above normal rate. As the desired temperature is approached but before it is reached relay 6! is actuated thereby deenergizing relay i28 and heater C is deenergized and heater is is transferred from source voltage to the chosen lower voltage of coil H9 and the rate at which the temperature increases toward the desired value is materially decreased.

In some cases it may be unnecessary to employ heater C at all. This may be done by setting switch arm I23 on contact I21. When this is done heater C is effectively removed from consideration and the desired temperature is approached while warming up at a slower rate which may be preferable in some instances.

As will occur to those skilled in the art, various different arrangements and combinations of the principles described above may be employed without departing from the true spirit and scope of the invention and we therefore do not wish to limit our invention to the particular arrangement described.

What we claim as new and desire to secure by Letters Patent of the United States is:

l. A control circuit for a constant temperature electrically heated device comprising an A. C. source, oscillator means for generating radio frequency oscillations adapted to be energized from said source, a temperature responsive alternating current bridge circuit adapted to be affected by the temperature of said heating device, first circuit means for energizing said bridge from said oscillator means, second circuit means adapted to be energized from said oscillator means and responsive to the output voltage of said bridge for producing an output current whose value de creases as said bridge becomes unbalanced and for producing a voltage dependent in magnitude upon both the degree and direction of unbalance of said bridge, a pluralit of electrical heating .elements, third circuit means including grid controlled vapor discharge tubes adapted to energize a portion of said heating elements from said A. C.

source to an extent dependent upon the voltage r balanced a predetermined amount because of excessive temperatures, means including said relays adapted to energize a second portion of said heating elements from said A. C. source at one heatproducing rate for temperatures within a predetermined range of temperatures on each side of bridge balance and at a higher rate for temperatures below said predetermined range and deenergizing said second portion for temperatures above said predetermined range, and means including said relays adapted to energize the remaining portion of said heating elements only when the temperature of said device is below said predetermined range thereby allowing rapid approach of temperature to said predetermined range and a slower rate of approach towards bridge balance within said predetermined range so as to permit rapid return to desired conditions without overcompensation.

2. In a control circuit, the combination of a source of alternating-current electric power, a source of unmodulated radio-frequency voltage, a modulator to provide an amplitude-modulated radio-frequency current having a carrier frequency and phase determined by said unmodulated radio-frequency voltage and a modulation envelope frequency and phase determined by said alternating current electric power, an alternating-current bridge circuit, connections to energize said bridge circuit with said amplitude modulated radio-frequency current, an amplifier to amplify the unbalance voltage of the bridge circuit to provide a first error voltage of amplitude-modulated radio frequency the carrier phase of which is determined by the direction of bridge unbalance, circuit means to add to said first error voltage an unmodulated radio-frequency voltage having an amplitude greater than the maximum amplitude of the first error voltage to obtain a second error voltage, a demodulator to provide a third error voltage the frequency and phase of which are determined by the modulation envelope of said second error voltage, and circuit means to add t said third error voltage a voltage having a frequency determined by said alternating-current electric power and a phase angle of therewith to obtain a fourth error voltage having a frequency determined by said alternating-current electric power and a phase determined by the degree and direction of unbalance of said bridge circuit.

PHILIP C. MICHEL. THEODORE A. RICH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,316,875 Laboulais Apr. 20, 1943 2,410,295 Kuehni et a1 Oct. 29, 1946 2,419,266 Kliever et al Apr. 22, 1947 2,423,864 Washburn et al. July 15, 1947 2,438,288 Jacobsen et a1 Mar. 23, 1948 

